This invention relates to a travelling finisher apparatus for making a road surface layer of a bituminous compound material, said apparatus comprising a first precompacting and levelling screed carried by a screed frame, and optionally a second levelling screed connected to vibratory drive means.
From a paper read by Mr. M. Blumer at a symposium on modern soil and asphalt surface layer compaction techniques held on Nov. 22 and 23 at Biel, Switzerland, it has become known that the service life of a road surface layer consisting of a bituminous compound material (asphalt surface layer) depends largely on the reduction of the voids therein to the smallest possible volume. The determining factor of the void volume is the degree of compaction which may be measured in drill core samples by means of a Marshall test body. The degree of compaction is the specific weight of the core sample as related to the specific weight of the Marshall test body. According to the findings explained in this paper, modern requirements can only be met by asphalt surface layers having a degree of compaction of at least about 98 percent. In practice a compaction degree of this magnitude has been achieved by providing a travelling finisher apparatus employed for laying down the surface layer with a hydraulically operated compactor bar adjacent the leading edge of the levelling screed, cooperating therewith to precompact the road surface layer to a maximum compaction degree of 93.5%. The compactor bar streaks the compound material to the proper level and compacts it by a ramming action as well as by means of its oblique leading face effective to compress the material to a reduced cross-section. The subsequently acting levelling screed is effective to close and to smoothen the surface. The subsequently required final compaction to a compaction degree of at least 98% requires the employment of road rollers immediately following the finisher apparatus. This purpose is achieved by means of static smooth-walled rollers and/or vibratory rollers, which may have to travel as much as ten times over each surface unit of the road surface layer. As the rolling operation has to be carried out synchronously with the travel of the finisher apparatus, the wide lanes laid down in large-capacity roadbuilding operations require the simultaneous employ of a plurality of rollers for enabling the requisite roller compaction to be carried out synchronously with the finisher travel with the road surface layer still in the plastic state. This final compaction is usually carried out with static pressures of about 3 to 12 kp/cm.sup.2.
In finisher apparatus known from DE-OS Nos. 17 84 633 and 17 84 634, the second levelling screed is formed as a trailing vibratory compactor provided with vibratory drive means. The compactor contacts the precompacted surface layer with a skid-shaped vibrator plate extending in the travelling direction over a length corresponding approximately to one half of the travel path width. Mounted on the vibrator plate are rotary driven shafts carrying eccentric weights for generating pulsating forces in all directions in planes extending perpendicular to the shafts. The maximum downward directed resultant force obtainable by this vibration system corresponds to no more than twice the total weight of the compactor. A greater resultant force would cause the compactor to start jumping, which would result in damage at least in the surface area of the road cover layer. As the resultant force available for the compaction process is thus limited and is moreover distributed over the large surface of the vibrator plate, the specific surface unit load is far too small to permit a compaction degree of for example 98% to be obtained thereby. Although in both references cited above it is emphasized that the compaction degree obtained is so high as to render subsequent roller compaction unnecessary, it has been found in practice that the actually obtainable compaction may just barely suffice in the case of poured asphalt, whereas in the case of normal compound layers subsequent roller compaction is absolutely necessary. It is also obvious that with this compactor, i.e. with the large area of the vibrator plate it is impossible to obtain the specific surface unit loads achieved in the case of roller compaction by the substantially linear contact area of the roller. In addition it is to be noted that due to the employed drive system with rotating eccentric masses, the pulsating forces transmitted from the vibrator plate to the surfacing layer are not restricted to vertically directed forces, but also include forces acting in the travel direction or obliquely thereto, such forces being undesirable in any case.